My boat has two fuel tanks. The one I've been using is a 40 gallon aluminum tank. I just replaced a 25 gallon rusty steel tank with an aluminium version I had built. I have a main engine and a generator. I can connect the main to the 40 gallon tank and then generator to the 25 gallon tank, but if I had my choice I'd be able to run either motor from either tank. If that's too complex, how should I set it up so I can transfer fuel either direction if needed?